Every new parent longs for the thrill of their child's first word but this was a joy Sarah Harris never thought she would experience when her son Joe was diagnosed with an extremely rare metabolic disorder at the age of two.

Five years later, however, the boy stunned his family and the medical world with a message that proved that he had secretly taught himself to read.

Sarah told BBC News Online: "He picked up a piece of jigsaw.

"He gave it to me and said 'G', which was written on it. I thought it was a fluke, but gave him another letter and he said that as well. I started typing out one syllable for him and he could say that too."

From this incredible beginning his mother has gradually helped her son open the door to a new world of communication, painstakingly piecing together a shared language syllable by syllable.

Painstaking progress

With practise Joe and Mrs Harris were able to communicate through text messages, breaking through the years of frustrating silence between mother and son.

"He has made so much progress. The doctors said he would never be able to co-ordinate himself enough to use the mouse on the computer, but he has. He learns so quickly and he wants to communicate so badly."

The breakthrough means that whereas once Joe could only mouth a single syllable to express himself, now he can type a sentence.

"His condition means that he displays a lot of the symptoms of autism and he can be very single-minded," said Mrs Harris.

Now Joe's family are hoping to raise enough cash to buy him a computer which will enable Joe to communicate verbally by speaking what he types in the voice of a six-year-old child.

Work colleagues of Mrs Harris have already raised more than £1,500 for the Hertfordshire youngster.

Rare condition

"Hartnup Syndrome" is a very rare metabolic disorder, which is inherited as a recessive gene and only becomes apparent if carried by both parents.

The condition is so rare that when Joe was first diagnosed medics asked his parents if they could possibly be related.

When the couple traced back their family tree they found that they actually came from the same small village.

The illness is caused by a defect in amino acids in the small intestine and kidney and can lead to mental retardation, co-ordination problems; gastrointestinal, skin and central nervous system abnormalities.

Suffers can also be smaller than normal in height.

The condition was named after the family in which it was first discovered in 1956 in London. Four of the eight family members were sufferers.

For many parents like Mrs Harris the first inkling that something is wrong is their child's urine. Because Hartnup leads to toxicity in the gut the urine is unusually foul smelling.

At the age of two after a barrage of tests doctors finally diagnosed Hartnup, but because the syndrome is so rare the Harris family had to do a lot of the research into the condition themselves.

Mrs Harris said she had now reconciled herself to the fact that Joe will probably never be able to say more than one syllable orally, although she said his capacity to learn through text is improving by the day.